When the propagation environment where the sound pulse propagates is the underwater, for example, a sonar is widely used as a method for probing objects existing in the underwater. That is, the distance and direction of an object are acquired through sweeping the underwater by using a sonic beam and receiving a reflected wave from the object that comes within the sonic beam. For extending the probing distance, it is necessary to use a low frequency sonic wave (e.g., 500 Hz) of a small absorption attenuation. However, when the frequency of the sonic wave is lowered, the sonic beam becomes widened and further increases an influence by reflections at the sea surface and the seabed, thereby making it difficult to probe the object.
Therefore, recently, there has been an increasing interest in a new object probing method using phase conjugated waves and time reversal waves.
C. Prada et al. perform so-called the time reversal processing which receives backward scattered waves scattered from an object existing in the water towards a sound source side and reverses the time axis of the received wave signal, and re-radiates the time reversal signal into the water. The re-radiated sonic wave converges to the position of the object. The converged sonic wave generates a reflection wave stronger than before. The reflected wave is received, and the time reversal processing is performed thereon again to be re-radiated. The sonic wave strongly converges to the position of the object.
Through repeating the above-described operation, it is possible to specify objects in order from larger ones to smaller ones.
Non-Patent Document 1: C. Prada, S. Manneville, D. Spoliansky, and M. Fink, “Do composion of the time reversal operator: detection and selective focusing on two scatterers”, J. Acoust. Soc. Am. 99, 2067-2076 (1995)